Tension transients suppression during acceleration and/or deceleration for storage tape drive

ABSTRACT

In one embodiment, a system includes a controller configured to: determine a reference velocity at various intervals, receive or determine a secondary velocity based at least in part on a parameter associated with a drive mechanism motor, determine a value representative of a difference between the reference velocity and secondary velocity, adjust a speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and secondary velocity, receive/determine a primary velocity based on reading information encoded on a magnetic tape, at a switching point, make the reference velocity about equal to the primary velocity, after the switching point, determine the reference velocity at various intervals, after the switching point, determine a value representative of a difference between the reference velocity and primary velocity, and adjust the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and primary velocity.

BACKGROUND

The present invention relates to data storage systems, and more particularly, this invention relates to a system, method and computer program product for suppressing tension transients during acceleration and deceleration.

In magnetic storage systems, data is read from and written onto magnetic recording media utilizing magnetic transducers. Data is written on the magnetic recording media by moving a magnetic recording transducer to a position over the media where the data is to be stored. The magnetic recording transducer then generates a magnetic field, which encodes the data into the magnetic media. Data is read from the media by similarly positioning the magnetic read transducer and then sensing the magnetic field of the magnetic media. Read and write operations may be independently synchronized with the movement of the media to ensure that the data can be read from and written to the desired location on the media.

An important and continuing goal in the data storage industry is that of increasing the density of data stored on a medium. For tape storage systems, that goal has led to increasing the track and linear bit density on recording tape, and decreasing the thickness of the magnetic tape medium. However, the development of small footprint, higher performance tape drive systems has created various problems in the design of a tape head assembly for use in such systems.

In a tape drive system, magnetic tape is moved over the surface of the tape head at high speed. Usually the tape head is designed to minimize the spacing between the head and the tape. The spacing between the magnetic head and the magnetic tape is crucial so that the recording gaps of the transducers, which are the source of the magnetic recording flux, are in near contact with the tape to effect writing sharp transitions, and so that the read element is in near contact with the tape to provide effective coupling of the magnetic field from the tape to the read element.

BRIEF SUMMARY

In one embodiment, a system includes a magnetic head, a drive mechanism for passing a magnetic tape over the magnetic head, and a controller electrically coupled to the drive mechanism, the controller being configured to: determine a reference velocity at various intervals, receive or determine a secondary velocity based at least in part on a parameter associated with a motor of the drive mechanism, determine a value representative of a difference between the reference velocity and the secondary velocity, adjust a speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the secondary velocity, receive or determine a primary velocity based on reading information encoded on a magnetic tape, at a switching point, make the reference velocity about equal to the primary velocity, after the switching point, determine the reference velocity at various intervals, after the switching point, determine a value representative of a difference between the reference velocity and the primary velocity, and adjust the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity.

In another embodiment, a method includes determining a reference velocity at various intervals, receiving or determining a secondary velocity based at least in part on a parameter associated with a motor of the drive mechanism, determining a value representative of a difference between the reference velocity and the secondary velocity, adjusting a speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the secondary velocity, receiving or determine a primary velocity based on reading a servo track of the magnetic tape, at a switching point, making the reference velocity about equal to the primary velocity, after the switching point, determining the reference velocity at various intervals, after the switching point, determining a value representative of a difference between the reference velocity and the primary velocity, and adjusting the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity.

In yet another embodiment, a system includes a magnetic head, a drive mechanism for passing a magnetic tape over the magnetic head, and a controller electrically coupled to the drive mechanism, the controller being configured to: determine a reference velocity at various intervals, receive or determine a primary velocity based on reading a servo track of the magnetic tape, determine a value representative of a difference between the reference velocity and the primary velocity, adjust a speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity, receive or determine a secondary velocity based at least in part on a parameter associated with a motor of the drive mechanism, at a switching point, make the reference velocity about equal to the secondary velocity, after the switching point, determine the reference velocity at various intervals, after the switching point, determine a value representative of a difference between the reference velocity and the secondary velocity, and adjust the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the secondary velocity.

In a further embodiment, a method includes determining a reference velocity at various intervals, receiving or determining a primary velocity based on reading a servo track of the magnetic tape, determining a value representative of a difference between the reference velocity and the primary velocity, adjusting a speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity, receiving or determining a secondary velocity based at least in part on a parameter associated with a motor of the drive mechanism, at a switching point, making the reference velocity about equal to the secondary velocity, after the switching point, determining the reference velocity at various intervals, after the switching point, determining a value representative of a difference between the reference velocity and the secondary velocity, and adjusting the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the secondary velocity.

Any of these embodiments may be implemented in a magnetic data storage system such as a tape drive system, which may include a magnetic head, a drive mechanism for passing a magnetic medium (e.g., recording tape) over the magnetic head, and a controller electrically coupled to the magnetic head.

Other aspects and embodiments of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a schematic diagram of a simplified tape drive system according to one embodiment.

FIG. 1B is a schematic diagram of a tape cartridge according to one embodiment.

FIG. 2 illustrates a side view of a flat-lapped, bi-directional, two-module magnetic tape head according to one embodiment.

FIG. 2A is a tape bearing surface view taken from Line 2A of FIG. 2.

FIG. 2B is a detailed view taken from Circle 2B of FIG. 2A.

FIG. 2C is a detailed view of a partial tape bearing surface of a pair of modules.

FIG. 3 is a partial tape bearing surface view of a magnetic head having a write-read-write configuration.

FIG. 4 is a partial tape bearing surface view of a magnetic head having a read-write-read configuration.

FIG. 5 is a side view of a magnetic tape head with three modules according to one embodiment where the modules all generally lie along about parallel planes.

FIG. 6 is a side view of a magnetic tape head with three modules in a tangent (angled) configuration.

FIG. 7 is a side view of a magnetic tape head with three modules in an overwrap configuration.

FIG. 8 is a graphical representation of a velocity profile for a conventional velocity control loop system.

FIG. 9 is a graphical representation of a velocity profile according to one embodiment of the presently claimed invention.

FIG. 10 is a flow diagram of a method according to one embodiment.

FIG. 11 is a flow diagram of a method according to one embodiment.

FIG. 12 is a representation of a velocity control loop system according to one embodiment.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified.

The following description discloses several preferred embodiments of magnetic storage systems, as well as operation and/or component parts thereof.

In one general embodiment, a system includes a magnetic head, a drive mechanism for passing a magnetic tape over the magnetic head, and a controller electrically coupled to the drive mechanism, the controller being configured to: determine a reference velocity at various intervals, receive or determine a secondary velocity based at least in part on a parameter associated with a motor of the drive mechanism, determine a value representative of a difference between the reference velocity and the secondary velocity, adjust a speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the secondary velocity, receive or determine a primary velocity based on reading information encoded on a magnetic tape, at a switching point, make the reference velocity about equal to the primary velocity, after the switching point, determine the reference velocity at various intervals, after the switching point, determine a value representative of a difference between the reference velocity and the primary velocity, and adjust the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity.

In another general embodiment, a method includes determining a reference velocity at various intervals, receiving or determining a secondary velocity based at least in part on a parameter associated with a motor of the drive mechanism, determining a value representative of a difference between the reference velocity and the secondary velocity, adjusting a speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the secondary velocity, receiving or determine a primary velocity based on reading a servo track of the magnetic tape, at a switching point, making the reference velocity about equal to the primary velocity, after the switching point, determining the reference velocity at various intervals, after the switching point, determining a value representative of a difference between the reference velocity and the primary velocity, and adjusting the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity.

In yet another general embodiment, a system includes a magnetic head, a drive mechanism for passing a magnetic tape over the magnetic head, and a controller electrically coupled to the drive mechanism, the controller being configured to: determine a reference velocity at various intervals, receive or determine a primary velocity based on reading a servo track of the magnetic tape, determine a value representative of a difference between the reference velocity and the primary velocity, adjust a speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity, receive or determine a secondary velocity based at least in part on a parameter associated with a motor of the drive mechanism, at a switching point, make the reference velocity about equal to the secondary velocity, after the switching point, determine the reference velocity at various intervals, after the switching point, determine a value representative of a difference between the reference velocity and the secondary velocity, and adjust the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the secondary velocity.

In a further general embodiment, a method includes determining a reference velocity at various intervals, receiving or determining a primary velocity based on reading a servo track of the magnetic tape, determining a value representative of a difference between the reference velocity and the primary velocity, adjusting a speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity, receiving or determining a secondary velocity based at least in part on a parameter associated with a motor of the drive mechanism, at a switching point, making the reference velocity about equal to the secondary velocity, after the switching point, determining the reference velocity at various intervals, after the switching point, determining a value representative of a difference between the reference velocity and the secondary velocity, and adjusting the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the secondary velocity.

FIG. 1A illustrates a simplified tape drive 100 of a tape-based data storage system, which may be employed in the context of the present invention. While one specific implementation of a tape drive is shown in FIG. 1A, it should be noted that the embodiments described herein may be implemented in the context of any type of tape drive system.

As shown, a tape supply cartridge 120 and a take-up reel 121 are provided to support a tape 122. One or more of the reels may form part of a removable cartridge and are not necessarily part of the system 100. The tape drive, such as that illustrated in FIG. 1A, may further include drive motor(s) to drive the tape supply cartridge 120 and the take-up reel 121 to move the tape 122 over a tape head 126 of any type. Such head may include an array of readers, writers, or both. The tape drive may also include a driver mechanism 127 to supply current to the drive motor(s) 124. The driver mechanism 127 may be coupled to a controller 128 and the drive the drive motor(s) 124 via a cable.

Guides 125 guide the tape 122 across the tape head 126. Such tape head 126 is in turn coupled to a controller 128 via a cable 130. The controller 128, may be or include a processor and/or any logic for controlling any subsystem of the drive 100. For example, the controller 128 typically controls head functions such as servo following, data writing, data reading, etc. The controller 128 may operate under logic known in the art, as well as any logic disclosed herein. The controller 128 may be coupled to a memory 136 of any known type, which may store instructions executable by the controller 128. Moreover, the controller 128 may be configured and/or programmable to perform or control some or all of the methodology presented herein. Thus, the controller may be considered configured to perform various operations by way of logic programmed into a chip; software, firmware, or other instructions being available to a processor; etc. and combinations thereof.

The cable 130 may include read/write circuits to transmit data to the head 126 to be recorded on the tape 122 and to receive data read by the head 126 from the tape 122. An actuator 132 controls position of the head 126 relative to the tape 122.

An interface 134 may also be provided for communication between the tape drive 100 and a host (integral or external) to send and receive the data and for controlling the operation of the tape drive 100 and communicating the status of the tape drive 100 to the host, all as will be understood by those of skill in the art.

FIG. 1B illustrates an exemplary tape cartridge 150 according to one embodiment. Such tape cartridge 150 may be used with a system such as that shown in FIG. 1A. As shown, the tape cartridge 150 includes a housing 152, a tape 122 in the housing 152, and a nonvolatile memory 156 coupled to the housing 152. In some approaches, the nonvolatile memory 156 may be embedded inside the housing 152, as shown in FIG. 1B. In more approaches, the nonvolatile memory 156 may be attached to the inside or outside of the housing 152 without modification of the housing 152. For example, the nonvolatile memory may be embedded in a self-adhesive label. In one preferred embodiment, the nonvolatile memory 156 may be a Flash memory device, ROM device, etc., embedded into or coupled to the inside or outside of the tape cartridge 150. The nonvolatile memory is accessible by the tape drive and the tape operating software (the driver software), and/or other device.

By way of example, FIG. 2 illustrates a side view of a flat-lapped, bi-directional, two-module magnetic tape head 200 which may be implemented in the context of the present invention. As shown, the head includes a pair of bases 202, each equipped with a module 204, and fixed at a small angle α with respect to each other. The bases may be “U-beams” that are adhesively coupled together. Each module 204 includes a substrate 204A and a closure 204B with a thin film portion, commonly referred to as a “gap” in which the readers and/or writers 206 are formed. In use, a tape 208 is moved over the modules 204 along a media (tape) bearing surface 209 in the manner shown for reading and writing data on the tape 208 using the readers and writers. The wrap angle θ of the tape 208 at edges going onto and exiting the flat media support surfaces 209 are usually between about 0.1 degree and about 5 degrees.

The substrates 204A are typically constructed of a wear resistant material, such as a ceramic. The closures 204B made of the same or similar ceramic as the substrates 204A.

The readers and writers may be arranged in a piggyback or merged configuration. An illustrative piggybacked configuration comprises a (magnetically inductive) writer transducer on top of (or below) a (magnetically shielded) reader transducer (e.g., a magnetoresistive reader, etc.), wherein the poles of the writer and the shields of the reader are generally separated. An illustrative merged configuration comprises one reader shield in the same physical layer as one writer pole (hence, “merged”). The readers and writers may also be arranged in an interleaved configuration. Alternatively, each array of channels may be readers or writers only. Any of these arrays may contain one or more servo track readers for reading servo data on the medium.

FIG. 2A illustrates the tape bearing surface 209 of one of the modules 204 taken from Line 2A of FIG. 2. A representative tape 208 is shown in dashed lines. The module 204 is preferably long enough to be able to support the tape as the head steps between data bands.

In this example, the tape 208 includes 4 to 22 data bands, e.g., with 16 data bands and 17 servo tracks 210, as shown in FIG. 2A on a one-half inch wide tape 208. The data bands are defined between servo tracks 210. Each data band may include a number of data tracks, for example 1024 data tracks (not shown). During read/write operations, the readers and/or writers 206 are positioned to specific track positions within one of the data bands. Outer readers, sometimes called servo readers, read the servo tracks 210. The servo signals are in turn used to keep the readers and/or writers 206 aligned with a particular set of tracks during the read/write operations.

FIG. 2B depicts a plurality of readers and/or writers 206 formed in a gap 218 on the module 204 in Circle 2B of FIG. 2A. As shown, the array of readers and writers 206 includes, for example, 16 writers 214, 16 readers 216 and two servo readers 212, though the number of elements may vary. Illustrative embodiments include 8, 16, 32, 40, and 64 readers and/or writers 206 per array. A preferred embodiment includes 32 readers per array and/or 32 writers per array, where the actual number of transducing elements could be greater, e.g., 33, 34, etc. This allows the tape to travel more slowly, thereby reducing speed-induced tracking and mechanical difficulties and/or execute fewer “wraps” to fill or read the tape. While the readers and writers may be arranged in a piggyback configuration as shown in FIG. 2B, the readers 216 and writers 214 may also be arranged in an interleaved configuration. Alternatively, each array of readers and/or writers 206 may be readers or writers only, and the arrays may contain one or more servo readers 212. As noted by considering FIGS. 2 and 2A-B together, each module 204 may include a complementary set of readers and/or writers 206 for such things as bi-directional reading and writing, read-while-write capability, backward compatibility, etc.

FIG. 2C shows a partial tape bearing surface view of complimentary modules of a magnetic tape head 200 according to one embodiment. In this embodiment, each module has a plurality of read/write (R/W) pairs in a piggyback configuration formed on a common substrate 204A and an optional electrically insulative layer 236. The writers, exemplified by the write head 214 and the readers, exemplified by the read head 216, are aligned parallel to a direction of travel of a tape medium thereacross to form an R/W pair, exemplified by the R/W pair 222.

Several R/W pairs 222 may be present, such as 8, 16, 32 pairs, etc. The R/W pairs 222 as shown are linearly aligned in a direction generally perpendicular to a direction of tape travel thereacross. However, the pairs may also be aligned diagonally, etc. Servo readers 212 are positioned on the outside of the array of R/W pairs, the function of which is well known.

Generally, the magnetic tape medium moves in either a forward or reverse direction as indicated by arrow 220. The magnetic tape medium and head assembly 200 operate in a transducing relationship in the manner well-known in the art. The piggybacked MR head assembly 200 includes two thin-film modules 224 and 226 of generally identical construction.

Modules 224 and 226 are joined together with a space present between closures 204B thereof (partially shown) to form a single physical unit to provide read-while-write capability by activating the writer of the leading module and reader of the trailing module aligned with the writer of the leading module parallel to the direction of tape travel relative thereto. When a module 224, 226 of a piggyback head 200 is constructed, layers are formed in the gap 218 created above an electrically conductive substrate 204A (partially shown), e.g., of AlTiC, in generally the following order for the R/W pairs 222: an insulating layer 236, a first shield 232 typically of an iron alloy such as NiFe (permalloy), CZT or Al—Fe—Si (Sendust), a sensor 234 for sensing a data track on a magnetic medium, a second shield 238 typically of a nickel-iron alloy (e.g., 80/20 Permalloy), first and second writer pole tips 228, 230, and a coil (not shown).

The first and second writer poles 228, 230 may be fabricated from high magnetic moment materials such as 45/55 NiFe. Note that these materials are provided by way of example only, and other materials may be used. Additional layers such as insulation between the shields and/or pole tips and an insulation layer surrounding the sensor may be present. Illustrative materials for the insulation include alumina and other oxides, insulative polymers, etc.

The configuration of the tape head 126 according to one embodiment includes multiple modules, preferably three or more. In a write-read-write (W-R-W) head, outer modules for writing flank one or more inner modules for reading. Referring to FIG. 3, depicting a W-R-W configuration, the outer modules 252, 256 each include one or more arrays of writers 260. The inner module 254 of FIG. 3 includes one or more arrays of readers 258 in a similar configuration. Variations of a multi-module head include a R-W-R head (FIG. 4), a R-R-W head, a W-W-R head, etc. In yet other variations, one or more of the modules may have read/write pairs of transducers. Moreover, more than three modules may be present. In further approaches, two outer modules may flank two or more inner modules, e.g., in a W-R-R-W, a R-W-W-R arrangement, etc. For simplicity, a W-R-W head is used primarily herein to exemplify embodiments of the present invention. One skilled in the art apprised with the teachings herein will appreciate how permutations of the present invention would apply to configurations other than a W-R-W configuration.

FIG. 5 illustrates a magnetic head 126 according to one embodiment of the present invention that includes first, second and third modules 302, 304, 306 each having a tape bearing surface 308, 310, 312 respectively, which may be flat, contoured, etc. Note that while the term “tape bearing surface” appears to imply that the surface facing the tape 315 is in physical contact with the tape bearing surface, this is not necessarily the case. Rather, only a portion of the tape may be in contact with the tape bearing surface, constantly or intermittently, with other portions of the tape riding (or “flying”) above the tape bearing surface on a layer of air, sometimes referred to as an “air bearing”. The first module 302 will be referred to as the “leading” module as it is the first module encountered by the tape in a three module design for tape moving in the indicated direction. The third module 306 will be referred to as the “trailing” module. The trailing module follows the middle module and is the last module seen by the tape in a three module design. The leading and trailing modules 302, 306 are referred to collectively as outer modules. Also note that the outer modules 302, 306 will alternate as leading modules, depending on the direction of travel of the tape 315.

In one embodiment, the tape bearing surfaces 308, 310, 312 of the first, second and third modules 302, 304, 306 lie on about parallel planes (which is meant to include parallel and nearly parallel planes, e.g., between parallel and tangential as in FIG. 6), and the tape bearing surface 310 of the second module 304 is above the tape bearing surfaces 308, 312 of the first and third modules 302, 306. As described below, this has the effect of creating the desired wrap angle α₂ of the tape relative to the tape bearing surface 310 of the second module 304.

Where the tape bearing surfaces 308, 310, 312 lie along parallel or nearly parallel yet offset planes, intuitively, the tape should peel off of the tape bearing surface 308 of the leading module 302. However, the vacuum created by the skiving edge 318 of the leading module 302 has been found by experimentation to be sufficient to keep the tape adhered to the tape bearing surface 308 of the leading module 302. The trailing edge 320 of the leading module 302 (the end from which the tape leaves the leading module 302) is the approximate reference point which defines the wrap angle α₂ over the tape bearing surface 310 of the second module 304. The tape stays in close proximity to the tape bearing surface until close to the trailing edge 320 of the leading module 302. Accordingly, read and/or write elements 322 may be located near the trailing edges of the outer modules 302, 306. These embodiments are particularly adapted for write-read-write applications.

A benefit of this and other embodiments described herein is that, because the outer modules 302, 306 are fixed at a determined offset from the second module 304, the inner wrap angle α₂ is fixed when the modules 302, 304, 306 are coupled together or are otherwise fixed into a head. The inner wrap angle α₂ is approximately tan⁻¹ (δ/W) where δ is the height difference between the planes of the tape bearing surfaces 308, 310 and W is the width between the opposing ends of the tape bearing surfaces 308, 310. An illustrative inner wrap angle α₂ is in a range of about 0.5° to about 1.1°, though can be any angle required by the design.

Beneficially, the inner wrap angle α₂ may be set slightly less on the side of the module 304 receiving the tape (leading edge) than the inner wrap angle α₃ on the trailing edge, as the tape 315 rides above the trailing module 306. This difference is generally beneficial as a smaller α₃ tends to oppose what has heretofore been a steeper exiting effective wrap angle.

Note that the tape bearing surfaces 308, 312 of the outer modules 302, 306 are positioned to achieve a negative wrap angle at the trailing edge 320 of the leading module 302. This is generally beneficial in helping to reduce friction due to contact with the trailing edge 320, provided that proper consideration is given to the location of the crowbar region that forms in the tape where it peels off the head. This negative wrap angle also reduces flutter and scrubbing damage to the elements on the leading module 302. Further, at the trailing module 306, the tape 315 flies over the tape bearing surface 312 so there is virtually no wear on the elements when tape is moving in this direction. Particularly, the tape 315 entrains air and so will not significantly ride on the tape bearing surface 312 of the third module 306 (some contact may occur). This is permissible, because the leading module 302 is writing while the trailing module 306 is idle.

Writing and reading functions are performed by different modules at any given time. In one embodiment, the second module 304 includes a plurality of data and optional servo readers 331 and no writers. The first and third modules 302, 306 include a plurality of writers 322 and no readers, with the exception that the outer modules 302, 306 may include optional servo readers. The servo readers may be used to position the head during reading and/or writing operations. The servo reader(s) on each module are typically located towards the end of the array of readers or writers.

By having only readers or side by side writers and servo readers in the gap between the substrate and closure, the gap length can be substantially reduced. Typical heads have piggybacked readers and writers, where the writer is formed above each reader. A typical gap is 25-35 microns. However, irregularities on the tape may tend to droop into the gap and create gap erosion. Thus, the smaller the gap is the better. The smaller gap enabled herein exhibits fewer wear related problems.

In some embodiments, the second module 304 has a closure, while the first and third modules 302, 306 do not have a closure. Where there is no closure, preferably a hard coating is added to the module. One preferred coating is diamond-like carbon (DLC).

In the embodiment shown in FIG. 5, the first, second, and third modules 302, 304, 306 each have a closure 332, 334, 336, which extends the tape bearing surface of the associated module, thereby effectively positioning the read/write elements away from the edge of the tape bearing surface. The closure 332 on the second module 304 can be a ceramic closure of a type typically found on tape heads. The closures 334, 336 of the first and third modules 302, 306, however, may be shorter than the closure 332 of the second module 304 as measured parallel to a direction of tape travel over the respective module. This enables positioning the modules closer together. One way to produce shorter closures 334, 336 is to lap the standard ceramic closures of the second module 304 an additional amount. Another way is to plate or deposit thin film closures above the elements during thin film processing. For example, a thin film closure of a hard material such as Sendust or nickel-iron alloy (e.g., 45/55) can be formed on the module.

With reduced-thickness ceramic or thin film closures 334, 336 or no closures on the outer modules 302, 306, the write-to-read gap spacing can be reduced to less than about 1 mm, e.g., about 0.75 mm, or 50% less than standard LTO tape head spacing. The open space between the modules 302, 304, 306 can still be set to approximately 0.5 to 0.6 mm, which in some embodiments is ideal for stabilizing tape motion over the second module 304.

Depending on tape tension and stiffness, it may be desirable to angle the tape bearing surfaces of the outer modules relative to the tape bearing surface of the second module. FIG. 6 illustrates an embodiment where the modules 302, 304, 306 are in a tangent or nearly tangent (angled) configuration. Particularly, the tape bearing surfaces of the outer modules 302, 306 are about parallel to the tape at the desired wrap angle α₂ of the second module 304. In other words, the planes of the tape bearing surfaces 308, 312 of the outer modules 302, 306 are oriented at about the desired wrap angle α₂ of the tape 315 relative to the second module 304. The tape will also pop off of the trailing module 306 in this embodiment, thereby reducing wear on the elements in the trailing module 306. These embodiments are particularly useful for write-read-write applications. Additional aspects of these embodiments are similar to those given above.

Typically, the tape wrap angles may be set about midway between the embodiments shown in FIGS. 5 and 6.

FIG. 7 illustrates an embodiment where the modules 302, 304, 306 are in an overwrap configuration. Particularly, the tape bearing surfaces 308, 312 of the outer modules 302, 306 are angled slightly more than the tape 315 when set at the desired wrap angle α₂ relative to the second module 304. In this embodiment, the tape does not pop off of the trailing module, allowing it to be used for writing or reading. Accordingly, the leading and middle modules can both perform reading and/or writing functions while the trailing module can read any just-written data. Thus, these embodiments are preferred for write-read-write, read-write-read, and write-write-read applications. In the latter embodiments, closures should be wider than the tape canopies for ensuring read capability. The wider closures will force a wider gap-to-gap separation. Therefore a preferred embodiment has a write-read-write configuration, which may use shortened closures that thus allow closer gap-to-gap separation.

Additional aspects of the embodiments shown in FIGS. 6 and 7 are similar to those given above.

A 32 channel version of a multi-module head 126 may use cables 350 having leads on the same pitch as current 16 channel piggyback LTO modules, or alternatively the connections on the module may be organ-keyboarded for a 50% reduction in cable span. Over-under, writing pair unshielded cables can be used for the writers, which may have integrated servo readers.

The outer wrap angles α₁ may be set in the drive, such as by guides of any type known in the art, such as adjustable rollers, slides, etc. For example, rollers having an offset axis may be used to set the wrap angles. The offset axis creates an orbital arc of rotation, allowing precise alignment of the wrap angle α₁.

To assemble any of the embodiments described above, conventional u-beam assembly can be used. Accordingly, the mass of the resultant head can be maintained or even reduced relative to heads of previous generations. In other approaches, the modules may be constructed as a unitary body. Those skilled in the art, armed with the present teachings, will appreciate that other known methods of manufacturing such heads may be adapted for use in constructing such heads.

Conventional tape storage transport systems utilize a velocity control loop system that transports media (e.g. a magnetic tape) from a supply reel driven by a motor to a take up reel driven by another motor, and vice versa. The velocity control loop system generates a reference velocity (Vref) profile to which a feedback velocity is compared. The Vref profile may represent the desired or target velocity of the tape. The difference between Vref and the feedback velocity represents the velocity error (Verror), which the velocity transport system ultimately works to minimize.

Typically, there are at least two sources of feedback velocity: primary velocity (Vpri) and secondary velocity (Vsec). Vpri may be derived from location information encoded on the tape, e.g. servo patterns, longitudinal position (LPOS) patterns, etc. While Vpri is typically an accurate and direct measurement of tape velocity, it may not always be available. For example, Vpri may not be available and/or accurate when the tape is stopped, moving at very low speeds, or during acceleration and deceleration operations.

Additionally, Vsec may be used to estimate tape velocity. Vsec may be based on the amount of current driving the motors, for example. Vsec is generally not as accurate as Vpri; however Vsec is typically always available.

FIG. 8 illustrates a graphical representation of a velocity profile 800 for a conventional velocity control loop system. As shown, the Vref is a velocity profile starting from a velocity of zero and ramping up with a desired acceleration to a target velocity. During the acceleration, Verror=Vref−Vsec may be used by the velocity control loop system. While FIG. 8 shows the Vsec slightly lower than Vref, the Vsec may be higher, lower or equal to Vref.

With continued reference to FIG. 8, the feedback velocity may be switched from Vsec to Vpri when Vpri becomes available. However, at the switching point (Switching Point), Verror=Vref−Vpri will be large due to the difference in velocity profiles of Vpri and Vsec. Consequently, the large Verror will cause the velocity control loop system to make a large current correction to the motor, in turn resulting in a large tension transient to the tape transport system. Such tension transient occurs where the outgress of the tape on the outgoing reel and the ingress of tape on the incoming reel do not match, yielding a tape tension that is either too high or too low. These large tension transients are not desirable for thin media as it may stress the media, cause defects, and may even cause the media to break.

Similarly, such negative effects may occur during a deceleration operation when the velocity control loop system switches back to Vsec when Vpri become unavailable.

Embodiments of the present invention overcome the aforementioned drawbacks by providing a system, method and computer program product for improving suppressing tension transients during acceleration and deceleration. Accordingly, in some embodiments, when the feedback velocity is switched from Vsec to Vpri, the velocity control loop system may make Vref about equal to Vpri, such that Verror=Vref−Vpri is about zero. Subsequently, Vref may then continue to ramp up to a target velocity at a controlled, predetermined, calculated, etc. rate. Preferably, a system, method and computer program product may make the Vref exactly equal to the Vpri, such that the Verror=Vref−Vpri=0, thus preventing the introduction of any large tension transient to the velocity control loop system.

FIG. 9 shows an exemplary graphical representation of a velocity profile 900 according to an illustrative embodiment of the present invention. As an option, the present representations may be implemented in conjunction with features from any other embodiment listed herein, such as those described with reference to the other FIGS. Of course, however, such representations, and others presented herein, may be used in various applications and/or in permutations, which may or may not be specifically described in the illustrative embodiments listed herein.

As shown in FIG. 9, the Vref is a velocity profile starting from a velocity of zero and ramping up with a desired acceleration to a target velocity. During the acceleration, Verror=Vref−Vsec may be used by the velocity control loop system. While FIG. 9 again shows the Vsec slightly lower than Vref, the Vsec may be higher, lower or equal to Vref.

With continued reference to FIG. 9, the feedback velocity may be switched from Vsec to Vpri when Vpri becomes available. Thus, at the switching point, the Vref may be made about or exactly equal to the Vpri. Consequently, the Verror=Vref−Vpri may be about or exactly equal to zero.

Referring now to FIG. 10, a method 1000 for tension transient suppression is shown according to one illustrative embodiment. The method 1000 may be used, e.g., during a acceleration of a magnetic tape.

As an option, the present method 1000 may be implemented in conjunction with features from any other embodiments listed herein, such as those shown in the other FIGS. Of course, however, this method 1000 and others presented herein may be used in various applications and/or permutations, which may or may not be related to the illustrative embodiments listed herein. Further, the method 1000 presented herein may be carried out in any desired environment. Moreover, more or less operations than those shown in FIG. 10 may be included in method 1000, according to various embodiments.

As shown in FIG. 10 according to one approach, the method 1000 includes determining a reference velocity (Vref) at various intervals. See operation 1002. In one embodiment, the reference velocity may be determined at various intervals to achieve a desired acceleration profile.

As used herein, the intervals may be periodic, varied, received, calculated, scaled based on some criteria such as the tape velocity, etc. For example, in one embodiment, the reference velocity may be computed at periodic intervals. In another embodiment, the reference velocity may be calculated or received at various intervals after the switching point. In yet another embodiment, the reference velocity may be generated at various intervals after the switching point by a processor of the tape drive. In a further embodiment, the reference velocity may be computed at various intervals after the switching point using the same computation used to calculate the reference velocity before the switching point.

The method 1000 also includes receiving or determining a secondary velocity (Vsec) based at least in part on a parameter associated with a motor of the drive mechanism. See operation 1004. For example, Vsec can be received from or determined by a chip or processor that calculates Vsec using any known algorithm. In one embodiment, the parameter associated with a motor of the drive mechanism may include, but is not limited to a torque constant of the motor, a current level applied to the motor, an electromotive force (EMF) voltage of the motor, one or more reel motor counts, e.g., hall sensor counts, a friction value, etc., or other parameter as would be understood by one having skill in the art upon reading the current disclosure. In another embodiment, the parameter associated with a motor of the drive mechanism may be selected from the group consisting of a torque constant of the motor, a current level applied to the motor, and a friction value.

The method 1000 further includes determining a value representative of a difference between the reference velocity and the secondary velocity. See operation 1006. In one embodiment, the value representative of a difference between the reference velocity and the secondary velocity may be an error value, such as Verror.

In operation 1008, a speed of the motor of the drive mechanism is adjusted for minimizing the difference between the reference velocity and the secondary velocity.

In operation 1010, a primary velocity (Vpri) is received or determined based on reading information encoded on a magnetic tape. As used herein, said information may include, servo patterns, LPOS patterns, etc.

Operation 1012 of method 1000 includes, at a switching point, making the reference velocity about equal to the primary velocity. In one embodiment, the reference velocity is not computed at a switching point. For example, in another embodiment, the reference velocity may be set to within 5% of the primary velocity. In a preferred embodiment, the reference velocity may be set exactly equal to the primary velocity.

In yet another embodiment, the primary velocity may be higher than the secondary velocity at the switching point. In a further embodiment, the primary velocity may be lower than the secondary velocity at the switching point.

Additionally, the reference velocity is determined at various intervals after the switching point. The reference velocity may also be calculated or received at various intervals after the switching point in some embodiments.

The method 1000 also includes, after the switching point, determining a value, such as an error value, etc., representative of a difference between the reference velocity and the primary velocity. See operation 1014.

The method 1000 further includes adjusting the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity. See operation 1016. In one embodiment, the speed of the motor of the drive mechanism may be adjusted for minimizing the difference between the reference velocity and the primary velocity until a target velocity is achieved.

According to another embodiment, the method 1000 may further include adjusting a speed of a second motor based on the difference between the reference velocity and the secondary velocity before the switching point, and based on the difference between the reference velocity and the primary velocity after the switching point.

In yet another embodiment, the method 1000 may be applied during an acceleration operation. In a further embodiment, a desired acceleration of the tape may be between about 2 and about 15 m/s², and in an exemplary embodiment about 10 m/s².

Referring now to FIG. 11, a method 1100 for tension transient suppression is shown according to another illustrative embodiment. The method 1100 may be used, e.g., during a deceleration of a magnetic tape.

As an option, the present method 1100 may be implemented in conjunction with features from any other embodiments listed herein, such as those shown in the other FIGS. Of course, however, this method 1100 and others presented herein may be used in various applications and/or permutations, which may or may not be related to the illustrative embodiments listed herein. Further, the method 1100 presented herein may be carried out in any desired environment. Moreover, more or less operations than those shown in FIG. 11 may be included in method 1100, according to various embodiments.

As shown in FIG. 11 according to one approach, the method 1100 includes determining a reference velocity at various intervals. See operation 1102. In one embodiment, the reference velocity may be determined at various intervals to achieve a desired deceleration profile.

As used herein, the intervals may be periodic, may vary, may be received, may be calculated, may be scaled based on some criteria such as the tape velocity, etc. For example, in one embodiment, the reference velocity may be computed at periodic intervals.

The method 1100 also includes receiving or determining a primary velocity based on reading information encoded on a magnetic tape. See operation 1104. As used herein, said information may include, servo patterns, longitudinal position (LPOS) patterns, etc.

The method 1100 further includes determining a value representative of a difference between the reference velocity and the primary velocity. See operation 1106. In one embodiment, the value representative of a difference between the reference velocity and the primary velocity may be an error value. In operation 1108, a speed of the motor of the drive mechanism is adjusted for minimizing the difference between the reference velocity and the primary velocity.

In operation 1110, a secondary velocity is received or determined based at least in part on a parameter associated with a motor of the drive mechanism. In one embodiment, the parameter associated with a motor of the drive mechanism may include, but is not limited to a torque constant of the motor, a current level applied to the motor, an electromotive force (EMF) voltage of the motor, one or more reel motor counts, e.g., hall sensor counts, a friction value, etc., or other parameter as would be understood by one having skill in the art upon reading the current disclosure. In another embodiment, the parameter associated with a motor of the drive mechanism may be selected from the group consisting of a torque constant of the motor, a current level applied to the motor, and a friction value.

Operation 1112 of method 1100 includes, at a switching point, making the reference velocity about equal to the secondary velocity. In one embodiment, the reference velocity is not computed at a switching point. For example, the reference velocity may simply be set to about the secondary velocity. In one approach, the reference velocity may be set to within 5% of the secondary velocity. In a preferred embodiment, the reference velocity may be set exactly equal to the secondary velocity.

Depending on the operating conditions, the secondary velocity may be higher or lower than the primary velocity at the switching point.

The reference velocity may be determined at various intervals after the switching point. In one embodiment, the reference velocity may be calculated or received at various intervals after the switching point. In another embodiment, the reference velocity may be generated at various intervals after the switching point by a processor of the tape drive. In yet another embodiment, the reference velocity may be computed at various intervals after the switching point using the same computation used to calculate the reference velocity before the switching point.

The method 1100 also includes, after the switching point, determining a value, such as an error value, etc., representative of a difference between the reference velocity and the secondary velocity. See 1114.

The method 1100 further includes adjusting the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity. See operation 1116. In one embodiment, the speed of the motor of the drive mechanism may be adjusted for minimizing the difference between the reference velocity and the secondary velocity until a target velocity is achieved.

According to another embodiment, the method 1100 may further include adjusting a speed of a second motor based on the difference between the reference velocity and the secondary velocity after the switching point, and based on the difference between the reference velocity and the primary velocity before the switching point.

In yet another embodiment, the method 1100 may be applied during a deceleration operation.

Referring now to FIG. 12, an illustrative representation of a velocity control loop system 1200 is shown according to various illustrative embodiments. As an option, the present representations may be implemented in conjunction with features from any other embodiment listed herein, such as those described with reference to the other FIGS. Of course, however, such representations, and others presented herein, may be used in various applications and/or in permutations, which may or may not be specifically described in the illustrative embodiments listed herein. Moreover, the various components of the system 1200 may be standard components arranged and/or configured in accordance with the present description.

As shown in FIG. 12 according to one approach, a reference velocity (Vref) is compared to a feedback velocity (e.g., Vpri or Vsec) using summation logic 1202.

In one embodiment, the feedback velocity may be a primary velocity (Vpri). The primary velocity may be derived by logic 1214 based on information from a reader 1212 reading information encoded on a magnetic tape. As used herein, the reader may include a head-to-tip interface, a servo reader, etc. or other suitable reader as would be understood by one having skill in the art upon reading the current disclosure. Further, as used herein, the information encoded on a magnetic tape may include servo patterns, longitudinal position (LPOS) patterns or data, etc.

Additionally, in one approach, the primary velocity may be used as the feedback velocity when the Vpri becomes available, e.g. when the tape reaches an about constant velocity.

In another embodiment, the feedback velocity may be a secondary velocity (Vsec) derived by logic 1210. The secondary velocity may be derived based at least in part on a parameter associated with a motor 1208, in one approach. For example, said parameters may include, but are not limited to, a torque constant of the motor, a current level applied to the motor by the driver mechanism 1206, an electromotive force (EMF) voltage of the motor, one or more reel motor counts, e.g., hall sensor counts, a friction value, etc., or other parameter as would be understood by one having skill in the art upon reading the present disclosure, in another approach.

In yet another approach, the secondary velocity may be used as the feedback velocity during acceleration or deceleration operations, when the velocity is not valid or at low tape velocities, etc. In a further approach, velocity control loop system may switch the feedback velocity from the Vsec to Vpri when the Vpri becomes available.

Again with reference to FIG. 12, the output of the summation logic 1202 may be a velocity error (Verror). The velocity error may be the difference between the Vref and the feedback velocity, in one approach. For example, in one embodiment, the Verror may be the difference between the Vref and Vpri. In another embodiment, the Verror may be the difference between the Vref and Vsec.

Verror may then be sent to a control law 1204, which may be logic operating in or under control of the drive controller. The control law 1204 determines or calculates a value that may represent an amount of current, in one embodiment. For example, the value may represent an amount of current necessary to adjust a tape velocity to minimize Verror, in one approach. Accordingly, the calculated or determined value may then be sent to the driver mechanism 1206 which in turn may supply the motor 1208 with the amount of current necessary to adjust the tape velocity, according to another approach.

It will be clear that the various features of the foregoing systems and/or methodologies may be combined in any way, creating a plurality of combinations from the descriptions presented above.

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as “logic,” a “circuit,” “module,” or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a non-transitory computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the non-transitory computer readable storage medium include the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a portable compact disc read-only memory (e.g., CD-ROM), a Blu-ray disc read-only memory (BD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a non-transitory computer readable storage medium may be any tangible medium that is capable of containing, or storing a program or application for use by or in connection with an instruction execution system, apparatus, or device.

A computer readable signal medium may include a propagated data signal with computer readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a non-transitory computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device, such as an electrical connection having one or more wires, an optical fibre, etc.

Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fibre cable, RF, etc., or any suitable combination of the foregoing.

Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer, for example through the Internet using an Internet Service Provider (ISP).

Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart(s) and/or block diagram block or blocks.

It will be further appreciated that embodiments of the present invention may be provided in the form of a service deployed on behalf of a customer.

While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of an embodiment of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 

What is claimed is:
 1. A data storage system, comprising: a magnetic head; a drive mechanism for passing a magnetic tape over the magnetic head; and a controller electrically coupled to the drive mechanism, the controller being configured to: determine a reference velocity at various intervals; receive or determine a secondary velocity based at least in part on a parameter associated with a motor of the drive mechanism; determine a value representative of a difference between the reference velocity and the secondary velocity; adjust a speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the secondary velocity; receive or determine a primary velocity based on reading information encoded on a magnetic tape; at a switching point, make the reference velocity about equal to the primary velocity; after the switching point, determine the reference velocity at various intervals; after the switching point, determine a value representative of a difference between the reference velocity and the primary velocity; and adjust the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity.
 2. A system as recited in claim 1, wherein the reference velocity is computed at periodic intervals.
 3. A system as recited in claim 1, wherein the reference velocity is not computed at the switching point.
 4. A system as recited in claim 1, wherein the parameter associated with the motor is selected from a group consisting of torque constant of the motor, a current level applied to the motor, and a friction value.
 5. A system as recited in claim 1, wherein the controller is also configured to adjust a speed of a second motor based on the difference between the reference velocity and the secondary velocity before the switching point, and based on the difference between the reference velocity and the primary velocity after the switching point.
 6. A method, comprising: determining a reference velocity at various intervals; receiving or determining a secondary velocity based at least in part on a parameter associated with a motor of the drive mechanism; determining a value representative of a difference between the reference velocity and the secondary velocity; adjusting a speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the secondary velocity; receiving or determine a primary velocity based on reading a servo track of the magnetic tape; at a switching point, making the reference velocity about equal to the primary velocity; after the switching point, determining the reference velocity at various intervals; after the switching point, determining a value representative of a difference between the reference velocity and the primary velocity; and adjusting the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity.
 7. A method as recited in claim 6, wherein the reference velocity is computed at periodic intervals.
 8. A method as recited in claim 6, wherein the reference velocity is not computed at the switching point.
 9. A method as recited in claim 6, wherein the parameter associated with the motor is selected from a group consisting of torque constant of the motor, a current level applied to the motor, and a friction value.
 10. A method as recited in claim 6, further comprising adjusting a speed of a second motor based on the difference between the reference velocity and the secondary velocity before the switching point, and based on the difference between the reference velocity and the primary velocity after the switching point.
 11. A data storage system, comprising: a magnetic head; a drive mechanism for passing a magnetic tape over the magnetic head; and a controller electrically coupled to the drive mechanism, the controller being configured to: determine a reference velocity at various intervals; receive or determine a primary velocity based on reading a servo track of the magnetic tape; determine a value representative of a difference between the reference velocity and the primary velocity; adjust a speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity; receive or determine a secondary velocity based at least in part on a parameter associated with a motor of the drive mechanism; at a switching point, make the reference velocity about equal to the secondary velocity; after the switching point, determine the reference velocity at various intervals; after the switching point, determine a value representative of a difference between the reference velocity and the secondary velocity; and adjust the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the secondary velocity.
 12. A system as recited in claim 11, wherein the reference velocity is computed at periodic intervals.
 13. A system as recited in claim 11, wherein the reference velocity is not computed at the switching point.
 14. A system as recited in claim 11, wherein the parameter associated with the motor is selected from a group consisting of torque constant of the motor, a current level applied to the motor, and a friction value.
 15. A system as recited in claim 11, wherein the controller is also configured to adjust a speed of a second motor based on the difference between the reference velocity and the secondary velocity after the switching point, and based on the difference between the reference velocity and the primary velocity before the switching point.
 16. A method, comprising: determining a reference velocity at various intervals; receiving or determining a primary velocity based on reading a servo track of the magnetic tape; determining a value representative of a difference between the reference velocity and the primary velocity; adjusting a speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the primary velocity; receiving or determining a secondary velocity based at least in part on a parameter associated with a motor of the drive mechanism; at a switching point, making the reference velocity about equal to the secondary velocity; after the switching point, determining the reference velocity at various intervals; after the switching point, determining a value representative of a difference between the reference velocity and the secondary velocity; and adjusting the speed of the motor of the drive mechanism for minimizing the difference between the reference velocity and the secondary velocity.
 17. A method as recited in claim 16, wherein the reference velocity is computed at periodic intervals.
 18. A method as recited in claim 16, wherein the reference velocity is not computed at the switching point.
 19. A method as recited in claim 16, wherein the parameter associated with the motor is selected from a group consisting of torque constant of the motor, a current level applied to the motor, and a friction value.
 20. A method as recited in claim 16, further comprising adjusting a speed of a second motor based on the difference between the reference velocity and the secondary velocity after the switching point, and based on the difference between the reference velocity and the primary velocity before the switching point. 